Two processes, the de novo formation of vessels from differentiating endothelial cells or angioblasts in the developing embryo (vasculogenesis) and the growth of new capillary vessels from existing blood vessels (angiogenesis), are involved in the development of the vascular systems of animal organs and tissues.
Recent findings show that at the centre of the network regulating the growth and differentiation of the vascular system and its components, both during embryonic development and normal growth and in a wide number of pathological anomalies and diseases, lies the angiogenic factor known as "Vascular Endothelial Growth Factor" (=VEGF), along with its cellular receptors. (see, Breier, et al., Trends in Cell Biology 6: 454-456, 1996).
VEGF receptors are transmembrane receptor tyrosine kinases. They are characterized by an extracellular domain with seven immunoglobulin-like domains and an intracellular tyrosine kinase domain. Various types of VEGF receptor are known, e.g. VEGFR-1, VEGFR-2, and VEGFR-3.